1. Field of the Invention
The present invention relates to a copolyamide having a novel geometrical isomer ratio, to a process for producing the same and to a molding composition containing the novel copolyamide. The copolyamide and copolyamide composition of the present invention are useful for making molded articles such as machinery parts, electrical parts, and automobile parts.
2. Description of the Prior Art
Polyamides have excellent mechanical properties, chemical resistance, dyeability, and shapability. Utilizing these properties, polyamides have been used for fabricating various shaped articles such as textile products, films and molded parts. Various kinds of polyamides have been investigated for many purposes and required end use properties. Aliphatic polyamides such as polycaproamide (hereinafter referred to as nylon 6), polyhexamethyleneadipamide (hereinafter referred to as nylon 66) are now manufactured on a large industrial scale and used commercially. However, these aliphatic polyamides have the disadvantage that they are not always satisfactory when used for various machinery parts and automobile parts which require high rigidity and excellent dimensional stability.
These aliphatic polyamides have relatively low glass transition temperature. Therefore, the rigidity of molded articles produced from these aliphatic polyamides deteriorates significantly at higher circumambient temperatures, for example, temperatures above 60.degree. C., and the heat distortion temperature is relatively low. The humidity absorption of the aliphatic polyamides is relatively high, and, thus, the rigidity and dimensional stability show severe deterioration due to absorption of atmospheric humidity.
In order to improve the above-mentioned disadvantages of aliphatic polyamides, polyamides containing cyclic structural units such as aromatic and alicyclic units, in their main chain have been investigated. In particular, an alicyclic polyamide comprising a long chain aliphatic diamine having 11 to 13 carbon atoms and hexahydroterephthalic acid, has a high Young's modulus and is used as a raw material for producing textile fabrics having an excellent hand touchness, as described in, for example, Published Examined Japanese Patent Applications No. 29447/72 and No. 29047/72.
As hexahydroterephthalic acid exists originally in geometrical isomers, the mechanical and thermal properties of polyamides containing hexahydroterephthalic acid as one component, may depend on the trans isomer ratio of the hexahydroterephthalic acid residues in the polyamide. It is reported that heat supplied during melt-polymerization can result in isomerization of the hexahydroterephthalic acid residues in the polyamide to produce trans form hexahydroterephthalic acid residues in the polyamide, independent of the trans isomer ratio of the starting hexahydroterephthalic acid material.
However, the toughness of polyamides containing only trans isomer of the hexahydroterephthalic acid residues is poor, precluding their use in various molded articles.
Thus, improvement of the toughness of the polyamide principally consisting of an aliphatic diamine having 11 to 13 carbon atoms and hexahydroterephthalic acid is necessary while maintaining the inherent excellent rigidity, heat resistance and dimensional stability. This object can be achieved by manipulation of the trans isomer ratio of hexahydroterephthalic acid residues in the polyamide.